The present invention relates generally to communication systems, and more particularly, to a method and apparatus for spreading symbols in a communication system.
In current Code Division, Multiple Access (CDMA) communication systems, data to be transmitted is repeated within a frame based on a transmission rate. More particularly, voice encoded data exits a voice encoder (vocoder) at a particular transmission rate and is convolutionally encoded. As symbols exit the convolutional encoder, they are repeated a number of times, with the number being based on the current transmission rate. The result of simultaneously repeating symbols in such a way is to make the data rate over the air interface constant irrespective of the transmission rate. This is illustrated in FIG. 1. As shown, convolutional encoder 112 encodes input data bits 110 at a fixed encoding rate of one data bit to two data symbols (i.e., rate xc2xd) such that convolutional encoder 112 outputs data symbols 114 at twice the input data rate. In this case, the input bit xe2x80x9c0xe2x80x9d was convolutionally encoded resulting in symbol xe2x80x9c01xe2x80x9d. The output of encoder 112 is input into symbol repeater 116, where it is repeated a number of times based on the transmission rate (i.e., full, half, quarter, or eighth). In this particular example, the symbol was repeated once, indicating half-rate transmission.
One of the primary problems with the current transmission scheme is that the data repetition used for sub-rate transmission (particularly in the case of eighth-rate frames) results in a false determination of the transmission rate by the receiver. More particularly, because each symbol is repeated a number times, it is possible to generate very long strings of zeros. These strings of zeros are easily decodable by the full-rate decoder and have been observed to cause very low Symbol Error Rates (SERs) resulting in the frame falsely being decoded as a full-rate frame. Therefore a need exists for a method and apparatus for transmission within a communication system that reduces falsing within a communication system.